Said device and said method are known from DE 299 20 893 U1, for instance. With the known device, each suction port is allocated to a special negative pressure generation device. To control the negative pressure, the negative pressure generators can be turned on and off individually or in groups. In particular to eject the work pieces adhering to the conveyor belt, the negative pressure generator devices are switched off in the ejection area. The suction jets are positioned here at a short interval to the conveying side of the work piece. The distance between the conveying side of the work pieces and the suction jets is so small that only a small amount of false air is suctioned through the opening slits remaining on the two edges of the work piece. It forms a pseudo stationary negative pressure.
The layout of the known device is relatively complicated. A large number of negative pressure generator devices must be provided. This requires a relatively large amount of space. The design of the known device is not particularly compact. Due to the creation of stationary negative pressure, large work pieces require greater force per area unit to be drawn to the conveyor belt than smaller work pieces. Depending on the size of the particular work piece, the speed of the drive motor must be varied so that the conveying speed can be held constant. The generation of static negative pressure also drops more slowly, when shut off, for large work pieces than for small work pieces. Depending on the size and weight of the adhering work pieces, dropping off takes place after the expiration of varying periods of time after the negative pressure generator devices have been switched off. Planned and precise ejection of work pieces is not possible with the known device.
From U.S. Pat. No. 4,804,081 a device for suspended conveyance of work pieces is known. Also in this device, the suction jets are placed between two parallel conveyor belts. They are staggered at a short distance opposite a conveying side of the work pieces so that a small gap is created between the conveyor side of the work piece and the opening edge of the suction jet. To prevent the penetration of false air into the gap, cross bars are provided which subdivide the suction area between the conveyor belts into rectangular suction zones. When the work pieces are conveyed, stationary negative pressure is also created here which leads to the aforementioned disadvantages.
The object of the invention is to remove the disadvantages in accordance with the state of art. In particular it should be provided a device for suspended conveyance of non-magnetic work pieces which is easy and inexpensive to construct. An additional goal of the invention is to show a device and a method with which it is possible to precisely drop off work pieces of varying sizes and varying weights.
According to the device side invention, there is provided a means to ventilate the gap so that the work piece is held on the conveyor belt by a dynamic negative pressure which is created.
The term dynamic negative pressure means negative pressure which is created in accordance with the Venturi principle as the result of a current. The current flows from the ventilating device to the suction ports. The prerequisite for creation of dynamic negative pressure is that a sufficiently high volume current per time unit flows through the ventilating device to the gap and is suctioned off again through the suction ports. A sufficiently high current speed for the creation of negative pressure is then generated in the gap. The amount of negative pressure is calculated from the Bernoulli equation. Typical current speeds here are between 60 and 80 m/sec. The formation of dynamic negative pressure provided by the invention offers the advantage that, regardless of their size, work pieces can always be held on the conveyor belt with the same specific force per area unit. Precise dropping off is possible regardless of the size of the work piece. The work pieces can always be conveyed at the same speed without regulation of the power of a drive unit.
In optimised arrangement the suction ports on a wall pointing towards the work pieces of a negative pressure passageway running essentially parallel to the direction of conveyance of the work pieces.
Provision of a common negative pressure passageway makes it unnecessary to provide each of the suction ports with a special negative pressure generation device. The device can be operated with a single negative pressure generation device which is connected to the negative pressure passageway. It can be compact. Provision of a common negative pressure passageway also creates free constructive space which can be used for systems for precise dropping off of the work pieces.
In optimised arrangement the suction ports are created by suction jets in the wall. This makes it possible to equip the device with suction ports of a suitable suction cross section to meet the requirements.
In optimised arrangement the device is equipped with ventilating openings at regular intervals for ventilating on the side of the holding device pointing toward the work pieces so that a essentially uniform, dynamic negative pressure is created over the entire length of the gap. To ventilate the gap it is also possible, however, to provide grooves in the conveyor belt crosswise to the direction of conveyance, for example. In some cases the device for ventilating can also be provided from the work piece side, particularly when the work piece has breakthroughs of suitable size or a suitably irregular surface, for instance. The device for ventilating ensures that work pieces can always be held on the conveyor belt with the same force regardless of their size. The ventilating openings can be located on the wall of the negative pressure passageway pointing towards the work pieces. Each of the ventilating openings forms the end of a ventilating line reaching through the negative pressure passageway which can be connected with a ventilating passageway. The ventilating passageway may be connected for ventilating with the environment but it can also be pressurised with slight overpressure.
A device for control and/or regulation of the dynamic negative pressure is provided. This can be a device for control and/or regulation of a negative pressure source providing the negative pressure passageway with negative pressure. For instance, the device can be used to change the speed of a vacuum source used as negative pressure source. However, a means of controlling the amount of air passing through the ventilating passageway can also be provided. This makes the device particularly universal. For example, it can be easily adjusted to the particular requirements by setting an air shutter. Usually the negative pressure passageway is always under a specified negative pressure which is dimensioned so that the required current flow speed can be generated quickly in the gap when a work piece is placed on the conveyor belt.
Particularly for precise dropping off of the work pieces, a device can be provided for generation of a burst of compressed air interrupting the dynamic negative pressure when desired. The device for generation of a burst of compressed air can be provided along a specified area of the retaining device. The specified area may be a dropping off zone of the work pieces. The compressed air burst is directed from the negative pressure passageway through the suction ports to the gap. This permits a compact construction. The dynamic negative pressure can not only be interrupted by the dropping off of work pieces but also by blowing additional air into the specified area of the gap which achieves fast ejection of the work pieces. If air continues to be suctioned through the suction ports while the compressed air burst is being generated in the specified area, the amount of air added with the compressed air burst per time unit must be greater than the amount of air suctioned off through the suction ports.
In constructive design the device is equipped with compressed air jets arranged coaxially to the suction ports. The compressed air jets can be moved back and forth axially against the suction ports. A particularly advantageous design characteristic is that the suction ports can be closed with the compressed air jets. When closed, no more air can be suctioned from the gap in the negative pressure passageway through the suction ports.
With respect to construction it has been shown useful to position the compressed air jets in the negative pressure passageway. This permits a particularly compact design of the device.
To drop off the work piece it is also possible to close the suction ports. In this case a pistons which can be adjusted via compressed air can be used instead of the compressed air jets and which can close the suction ports when desired.
Another development is that a ventilating opening and, if necessary, a compressed air jet is allocated to each suction port. Naturally it is also possible that a different ratio of the number of suction ports to the number of ventilating openings and, if necessary, the compressed air jets can be chosen. The suction ports and the ventilating openings can be alternately arranged consecutively along the length of the retaining device. The aforementioned characteristics simplify the construction. This makes it inexpensive to manufacture the device.
The compressed air jets can be pressurised with compressed air to drop off the work pieces. Naturally it is possible to activate the compressed air jets in groups for precise dropping off of a work piece or to pressurise them with compressed air. A control device in accordance with the state of technology can be used for this.
It is considered particularly advantageous that the retaining device contains magnets (preferably electrically controllable) so that adherence of magnetic work pieces to the conveyor belt is controlled by a magnetic field. An additional advantageous constructive development is the placement of the negative pressure passageway between the magnets. Such a device is particularly universal. Not only can non-magnetic work pieces be conveyed and precisely dropped off but also magnetic work pieces.
The inventive device is further characterised by a procedure for suspended conveyance of work pieces, in particular plate-shaped work pieces such as metal sheets or plates so that a device is provided for ventilating the gap so that the work piece is retained on the conveyor belt by dynamic negative pressure created in the gap. By applying a dynamic negative pressure as used by the invention, expensive sealing measures familiar in state-of-technology construction can be omitted when stationary negative pressure is used.
An advantageous construction development is that the device is equipped at regular intervals with ventilating openings along the retaining device for ventilating so that a primarily uniform, dynamic negative pressure is available over the entire length of the gap. This ensures that the work pieces are always drawn to the conveyor belt with the same force at all points along the gap.
It is useful to interrupt the dynamic negative pressure with a burst of compressed air directed toward the gap to drop off the work pieces. The suggested procedure causes a sudden breakdown of the dynamic negative pressure. Work pieces adhering to the conveyor belt can thus be precisely dropped off. The compressed air burst can be directed from a negative pressure passageway through the suction ports to the gap. Provision of such a negative pressure passageway also contributes to the compact and simple construction of the device for performance of the procedure.
In addition, the dynamic negative pressure can be controlled and/or regulated with a device for ventilating the gap. To set a specified dynamic negative pressure, a specified amount of air can be fed to the gap. The device for ventilating can be equipped with ventilating openings on the wall of the negative pressure passageway pointing towards the work pieces. Each ventilating opening can be the end of a ventilating line covering the negative pressure passageway. The ventilating openings can be connected with a ventilating passageway. The dynamic negative pressure can be adjusted with a device for control and/or regulation. This can be a device with which the ventilating of the gap is controlled and/or regulated.
It is advantageous that the compressed air jets move axially in the direction of the suction ports when pressurised with compressed air. The suction ports can be closed preferably with the compressed air jets when the compressed air jets are pressurised with compressed air. The aforementioned procedural steps contribute to the particularly quick and efficient dispersion of the dynamic negative pressure.
The burst of compressed air is preferably generated in a specified group of consecutive suction ports. This makes it possible to precisely drop off particularly large plate-shaped work pieces. In addition, at least one additional suction port upstream of the group of suction ports can be closed to drop off the work piece. The suction port can be closed, for example, with a piston which can be pressurised with compressed air. This measure contributes to the avoidance of the occurrence of an undesired dynamic negative pressure around the edges of the dropping off zone.